java 如何在java中为单个键添加多个值
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how to add multiple values to single key in java
提问by Ock
How can I create a map that can extend its list of value. For example I have a map which contains: key A and list of values B {1,2,3}. Then at some point during the program running, I would like to add more value, ex: 4 to list B. Now my new map should be something like: (A,{1,2,3,4}). How can I do that?
如何创建可以扩展其值列表的地图。例如，我有一张地图，其中包含：键 A 和值列表 B {1,2,3}。然后在程序运行的某个时刻，我想添加更多值，例如：4 到列表 B。现在我的新地图应该是这样的：(A,{1,2,3,4})。我怎样才能做到这一点？
回答by dasblinkenlight
Since Map<K,V>maps one key to one value, you need Vto be of type List<T>:
由于Map<K,V>将一个键映射到一个值，因此您需要V具有以下类型List<T>：
Map<String,List<Integer>> multiMap = new HashMap<>();
One consequence of having a type like that is that now the process of adding an item to a key consists of two steps: you must check if the list for the key exists, and then either add a new item to an existing list, or add a new list with a single item:
拥有这样的类型的一个后果是，现在向键添加项的过程包括两个步骤：您必须检查键的列表是否存在，然后向现有列表添加新项，或添加包含单个项目的新列表：
List<Integer> current = multiMap.get(key);
if (current == null) {
current = new ArrayList<Integer>();
multiMap.put(key, current);
}
current.add(val);
If you use Java 8, you can use computeIfAbsentto greatly simplify the code:
如果你使用Java 8，你可以使用computeIfAbsent来大大简化代码：
multiMap.computeIfAbsent(key, k -> new ArrayList<>()).add(value);
回答by Vinod
Add new Integer using new temp variable, Check with this explains might be help.
使用新的临时变量添加新的整数，检查这个解释可能会有所帮助。
Map<String, List<Integer>> map = new HashMap<String, List<Integer>>();
List<Integer> list = new ArrayList<Integer>();
list.add(1);
list.add(2);
list.add(3);
map.put("A", list);
map.forEach((key,value)->System.out.print(key + " : "+
value));
List<Integer> newList = (ArrayList<Integer>)map.get("A");
newList.add(4);
System.out.println();
map.put("A", newList);
map.forEach((key,value)->System.out.print(key + " : "+
value));
回答by Sergey Prokofiev
Such type of collection already implemented. Take a look at Multimap from Guava. Don't invent the bike from the scratch.
这种类型的收集已经实施。看看来自 Guava 的 Multimap。不要从头开始发明自行车。
Hope it helps!
希望能帮助到你！
回答by Norbert
A more generic solution is to have a MultiMap class which provides any Collection type as value e.g. ArrayList: I found this in a Java-Book weeks ago. Example:
一个更通用的解决方案是拥有一个 MultiMap 类，它提供任何 Collection 类型作为值，例如 ArrayList：我几周前在 Java-Book 中发现了这个。例子：
class MultiMap<K, V>
{
private final Map<K,Collection<V>> map = new HashMap<>();
public Collection<V> get(K key)
{
return map.getOrDefault(key, Collections.<V> emptyList());
}
public void put(K key,V value )
{
map.computeIfAbsent( key, k -> new ArrayList<>()).add( value );
}
}
Usage may be: (the map value is a ArrayList of Strings)
用法可能是：（映射值是字符串的 ArrayList）
MultiMap<Integer,String> map = new MultiMap<>();
int id = 34;
System.out.println(map.get(id)); // creates an ArrayList<String> for key 34
map.put(id,"first value for 34");
System.out.println(map.get(id)); // [first value for 34]
map.put(id,"second");
System.out.println(map.get(id)); // [first value for 34, second]
Hope this helps...
希望这可以帮助...
回答by Alekya
public static void main(String[] args) {
Map<String, List<String>> map = new HashMap<String, List<String>>();
List<String> valSetOne = new ArrayList<String>();
valSetOne.add("ABC");
valSetOne.add("BCD");
valSetOne.add("DEF");
List<String> valSetTwo = new ArrayList<String>();
valSetTwo.add("CBA");
valSetTwo.add("DCB");
map.put("FirstKey", valSetOne);
map.put("SecondKey", valSetTwo);
for (Map.Entry<String, List<String>> entry : map.entrySet()) {
String key = entry.getKey();
List<String> values = entry.getValue();
System.out.println("Value of " + key + " is " + values);
}
}
}
You can use Set or List based on your requirement.This is a simple method of having single key with multiple values.
您可以根据您的要求使用 Set 或 List。这是一种具有多个值的单个键的简单方法。
回答by SanSan-
Java 8. Stream API.
Java 8. 流 API。
For example we have collection of Users as List:
例如，我们将用户集合作为列表：
[{ name: "qwer", surname: "asdf", type: "premium"}, { name: "ioup", surname: "vmnv", type: "usual"}, { name: "rtyr", surname: "zxc", type: "premium"}]
and we need to add multiple Users to Map where key is ?type?
并且我们需要将多个用户添加到 Map 其中键是？type？
Map<String, List<User>> map = collection.stream().collect(Collectors.groupingBy(User::getType, Collectors.mapping(Function.identity(), Collectors.toList())))
To process this map we can make new stream
为了处理这张地图，我们可以制作新的流
map.entrySet().stream().{map|reduce|& other things}...
